A PLL circuit is a circuit that compares the phase of an output signal of a VCO with the phase of a reference signal, and feeds the comparison result back to the VCO, to stabilize the oscillation frequency of the VCO. In a case where a signal with a frequency that steeply changes like sawtooth waves is to be output, the PLL circuit cannot follow the set waveform at a time point when the frequency changes steeply, and it takes time for the PLL circuit to again output a correct frequency. To solve this problem by a conventional technique, a D/A converter (DAC) is connected to a control terminal of a VCO via a switch, the switch is put into an ON-state at a time point when the frequency steeply changes, and thus the output of the DAC is supplied to the control terminal of the VCO. In this manner, the time required for the PLL to again output a correct frequency is shortened (see Non-Patent Document 1, for example).